The present invention relates to a data preservation technology on a vehicle-mounted magnetic disk device. More particularly, it relates to a vehicle-mounted information processing apparatus that includes a magnetic disk device having a high environment-temperature resistant capability suitable for a car-navigation system.
In recent years, in accompaniment with the implementation of downsizing and high performance of computers, an even smaller-sized and even larger-capacity magnetic disk device has been developed. Also, in a vehicle-mounted information processing apparatus such as a navigation system, it is requested to have not only information such as map information and various types of landmark information, but also an easy-to-use AV function. The existence of this request is now promoting the employment of a magnetic disk device as the information storage that is capable of performing large-capacity and high-speed information accumulation/reproduction, to say nothing of exhibiting high-speed access performance.
By the way, as a record/reproduction surface of the recent magnetic disk, a magnetic thin metal-film that is ten and several nm thick and on-average about 10 nm in particle-diameter has been employed in order to implement several-tens-of-Gbit record density per square inch. As a result, it is becoming increasingly difficult to ensure stability against the heat. For example, the so-called “thermal decay” becomes likely to occur. Here, the thermal decay is a phenomenon that simply causing the high-temperature state to persist results in a decline in the magnetization on the disk surface.
FIG. 9 illustrates the relationship of a residual magnetization with respect to an elapsed time after recording the magnetization, where the environment (conservation) temperature is selected as the parameter. As indicated by a bold line 300-1, there are some cases where, in the data recorded in an environment at about 80° C., an about 15% residual-magnetization decline (i.e., reproduced-amplitude decline) occurs 1000 hours after.
FIG. 10 illustrates the Raw BER (i.e., bit error rate before error correction) with respect to the reproduced amplitude (i.e., residual magnetization). As indicated by a bold line 300-3, there are some cases where, if the residual magnetization declines by about 15%, the Raw BER declines by as much as about 3 orders. Accordingly, a case is likely to occur where even an error correction circuit cannot make the correction.
In, e.g., a HDD built in a personal computer (: PC) or a server system, the upper limit of its operation environment is limited to substantially 50° C. This condition makes the problem of the “thermal decay” comparatively allowable. In the vehicle-mounted information processing apparatus such as the navigation system, however, the temperature within a car left under the blazing sun in summer rises up to the above-described temperature of about 80° C. Consequently, applying the PC-used HDD directly into the navigation system results in even a possibility of causing a fatal problem such as data loss after use for a constant time-period.
Incidentally, there exists JP-A-2001-307450 as a literature that describes a magnetic disk device to be mounted in the car-navigation system. In this literature, in the magnetic disk device that employs a load/unload mechanism as the head saving mechanism, an occurrence of emergency unload operation is prevented which is attributed to a variation in the power-source voltage at the time of starting the engine. This prevention is performed in order to enhance reliability of the vehicle-mounted magnetic disk device.